1. Field of the Invention
The present invention relates to an optical scanning apparatus used in a printer, and more particularly, to an optical scanning apparatus capable of improving bowing of scan lines on a surface to be scanned which occurs when a beam of light emitted from a light source is obliquely incident on a polygon mirror in a sub-scanning direction. In addition, the present invention relates to an optical scanning apparatus which can be used as a tandem optical scanning apparatus in which a plurality of beams of light are simultaneously focused on a plurality of organic photoconductors.
2. Description of the Related Art
In general, an optical scanning apparatus such as a laser scanning unit is applied to an image forming apparatus for printing an image on a sheet of paper such as found in a copier, printer, facsimile, or the like. An optical scanning apparatus is used to scan a beam of light emitted from a light source such as a laser diode on a photosensitive medium of the image forming apparatus to form an electrostatic latent image on the photosensitive medium.
FIGS. 1A and 1B are diagrams illustrating a sub-scanning direction of a scanning optical system disclosed in Japanese Laid-Open Patent Application No. 2002-333590, and show a light source portion and an image producing portion, respectively. In addition, FIG. 2 is a diagram describing a main scanning direction of the scanning optical system shown in FIGS. 1A and 1B.
Referring to FIGS. 1A, 1B, and 2, the scanning optical system has a light source portion 10 generating a laser beam or a plurality of laser beams, a cylindrical lens 13 for converging the beams emitted from the light source portion 10, a polygon mirror 20 as a deflector for reflecting and deflecting the laser beams converged by the cylindrical lens 13, and an image focusing system causing the light beams reflected by the polygon mirror 20 to form spots on surfaces to be scanned.
The light source portion 10 of the scanning optical system has two semiconductor lasers 11 which are light sources, and two collimating lenses 12 for converting beams emitted from the semiconductor lasers 11 into parallel beams. The cylindrical lens 13 serves as an anamorphic optical element for converging two beams L1 and L2 made parallel beams by the collimating lenses 12 in a sub-scanning direction.
The image focusing system has a scanning lens 30 consisting of a first lens 31 disposed on the polygon mirror 20 side and a second lens 32 disposed on the surface to be scanned side, and a correction lens 50 disposed to be closer to the surface to be scanned than the scanning lens 30 for correcting bowing of the scan lines
Here, at least one convex surface included in the scanning lens 30 is a toric surface having a strong refractive power in the sub-scanning direction, and one surface of the correction lens 50 is an anamorphic aspherical surface which is set so that the radius of curvature of the correction lens 50 in the sub-scanning direction can have nothing to do with the shape of a section of the correction lens 50 in the main scanning direction at a position away from the optical axis, and satisfies a formula −1.1<Rz2/Rz1<−0.3. Here, Rz1 is a radius of curvature of the toric surface in the sub-scanning direction, and Rz2 is a radius of curvature of the anamorphic aspherical surface in the sub-scanning direction.
In the above configuration, the two beams L1 and L2 emitted from the light source portion 10 are simultaneously deflected by the polygon mirror 20 which rotates about a rotation axis 20a. The two deflected beams L1 and L2 travel at a predetermined angle with the sub-scanning direction, and enter the scanning lens 30 consisting of the first lens 31 and the second lens 32. Thereafter, the beams exiting the scanning lens 30 are reflected by two pairs of mirrors 40 and 41, respectively, and are converged onto two photosensitive drums 60 to form beam spots thereon. The semiconductor lasers (light sources) 11 are on/off controlled and a predetermined electrostatic latent image is formed on the photosensitive drums 60.
With the above-described optical scanning system, it is possible to correct bowing of scan lines that occurs due to an aberration in the sub-scanning direction without changing the power distribution in the main scanning direction. In addition, in the tandem optical scanning apparatus used in a color laser printer or the like, the deflector can be used in common by simultaneously deflecting a plurality of beams with the single polygon mirror. Accordingly, the number of parts can be reduced, and the size of an apparatus can be smaller. In addition, since it is possible to make the polygon mirror thinner when the plurality of beams are caused to be incident on the polygon mirror obliquely with respect to the sub-scanning direction so that incident positions of the beams at the polygon mirror can be nearly the same, the manufacturing cost of the polygon mirror can be reduced.
However, when the laser beams are incident on the polygon mirror obliquely in the sub-scanning direction, scan lines, which are the traces of beam spots, are curved on the surface to be scanned. Such curving in scan lines is called bowing, and the bowing causes printing precision to be low, and deteriorates the quality of a printed image. Also, the bowing deteriorates reproduction of colors in a color laser printer having a tandem optical scanning apparatus.
In order to restrain the above bowing, a method of making the image producing optical system using two scanning lenses and one correcting lens has been used in the conventional scanning optical system described above. However, this method causes the number of parts to increase, and causes the cost of manufacture and assembly to increase, and, therefore, the benefit of using the polygon mirror in common is canceled out. In addition, the correction lens must be made as a plastic injection molded lens since the correction lens has the anamorphic aspherical surface having a lengthwise magnitude much greater than a thickness-wise magnitude, and it is difficult to achieve the required preciseness in the shape of the correction in manufacturing the plastic injection molded lens having such a shape.